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The Surrender Software Scientific image rendering for space scenes with the SurRender software Scientific image rendering for space scenes with the SurRender software R. Brochard, J. Lebreton*, C. Robin, K. Kanani, G. Jonniaux, A. Masson, N. Despré, A. Berjaoui Airbus Defence and Space, 31 rue des Cosmonautes, 31402 Toulouse Cedex, France [email protected] *Corresponding Author Abstract The autonomy of spacecrafts can advantageously be enhanced by vision-based navigation (VBN) techniques. Applications range from manoeuvers around Solar System objects and landing on planetary surfaces, to in -orbit servicing or space debris removal, and even ground imaging. The development and validation of VBN algorithms for space exploration missions relies on the availability of physically accurate relevant images. Yet archival data from past missions can rarely serve this purpose and acquiring new data is often costly. Airbus has developed the image rendering software SurRender, which addresses the specific challenges of realistic image simulation with high level of representativeness for space scenes. In this paper we introduce the software SurRender and how its unique capabilities have proved successful for a variety of applications. Images are rendered by raytracing, which implements the physical principles of geometrical light propagation. Images are rendered in physical units using a macroscopic instrument model and scene objects reflectance functions. It is specially optimized for space scenes, with huge distances between objects and scenes up to Solar System size. Raytracing conveniently tackles some important effects for VBN algorithms: image quality, eclipses, secondary illumination, subpixel limb imaging, etc. From a user standpoint, a simulation is easily setup using the available interfaces (MATLAB/Simulink, Python, and more) by specifying the position of the bodies (Sun, planets, satellites, …) over time, complex 3D shapes and material surface properties, before positioning the camera. SurRender comes with its own modelling tool, SuMoL, enabling to go beyond existing models for shapes, materials and sensors (projection, temporal sampling, electronics, etc.). SurRender is natively designed to simulate different kinds of sensors (visible, LIDAR, …). Additional tools are available for manipulating huge datasets (“giant textures”) used to store albedo maps and digital elevation models (up to 256 TB), or for procedural (fractal) texturing that generates high-quality images for a large range of observing distances (from millions of km to touchdown). We illustrate SurRender performances with a selection of case studies. We place particular emphasis on a Moon landing simulation we recently computed that represents 40 GB of data and a 900-km flyby. The SurRender software can be made available to interested readers upon request. Keywords: computer vision, navigation, image rendering, space exploration, raytracing Acronyms/Abbreviations removal. It is an asset when manoeuvers do not rely on ADS: Airbus Defence and Space the ground to avoid transmission delays and also because BRDF: Bidirectional Reflectance Distribution Function a very high reactivity maximizes the science return of the DEM: Digital Elevation Model mission during high relative velocity phases. GNC: Guidance, Navigation & Control Traditionally, Guidance, Navigation & Control (GNC) GPS: Global Positioning System filters hybridize the input from several sensors, such as GSD: Ground Sample Distance IMUs, GPS (in earth orbit), radars and altimeters. Over IMU: Inertial Measurement Unit the last decades, it has become evident that the addition JUICE: JUpiter ICy moons Explorer of measurements from Vision-Based Navigation (VBN) LIDAR: LIght Detection And Ranging systems greatly improves the performances of LSB: Least Significant Bit autonomous navigation solutions [1, 2, 3]. ADS has PDS: Planetary Data System developed a portfolio of VBN algorithms including PSF: Point Spread Function relative navigation, absolute navigation, model-based RAM: Random Access Memory navigation as well as perception and reconstruction R&T: Research and Technology techniques. These computer vision techniques rely on the VBN: Vision-Based Navigation availability of abundant test images to insure their development and validation. 1. Introduction Archival data from past missions can be used to test Solar System exploration probes as well as in-orbit algorithms but, when they are available, they are robotic spacecrafts require a high level of autonomy to generally sensor-specific, they lack exhaustiveness, perform their missions. The applications range from representativeness, and often ground truth references. manoeuvers and landing around Solar System objects, to Some experiments are carried out on ground, thanks to in-orbit robotic servicing missions, or space debris test benches equipped with robotic arms handling sensors V1.0 - October 2, 2018 Page 1 of 11 Scientific image rendering for space scenes with the SurRender software and target mock-up. Their representativeness is limited in content of a pixel is determined by casting several rays terms of range, spatial irradiance, available scenarios, originating from this pixel, and finding which real world inter alia, and these facilities have high operating costs, objects this ray encounters until it finally intersects with so that only a few test cases can be run. a light source (Sun/stars, planets or artificial light To assess performances and robustness of VBN sources). Physical optics-level effects, such as diffraction solutions, computer generated data are highly of the light by the aperture of the imaging instrument, are recommended as they allow testing solutions in taken into account at macroscopic level using a Point exhaustive cases and provide ground truth comparisons. Spread Function (PSF). The light flux is stochastically The objective of the image processing part of the VBN, sampled within the pixels (including the probability derived from computer vision technics, is often to extract density function defined by the PSF). information from the image geometry: track features, The raytracer handles multiple diffusions and match edges, etc. In this context, the capability of the reflections (This recursive raytracing technique is called algorithms to extract such information depends on pathtracing). Interaction of the light with the surface of notions such as texture, contrast, noise, which all come the scene objects is modelled in terms of Bidirectional from a broader field called radiometry. The same Reflectance Distribution Function (BRDF). The objects challenges exist for vision sensor design. themselves can take arbitrarily complex shapes. The Some simulator softwares are well-known to the geometry of Solar System objects are described by public: examples include the Celestia and Blender Digital Elevation Maps (DEM) and spheroid models software or rendering engines used for video game or by which are the basic input needed to calculate light animation studios. Although visually impressive, these scattering. Artificial objects are described by 3D meshes. simulators lack the realism needed for advanced image Providing the incoming irradiance is known, the processing techniques. Typically they are designed to image is naturally rendered in physical units: each pixel cope with the human vision, but they do not have the contains an irradiance value expressed in W/m². photometric accuracy that actual sensors are sensitive to. Providing the number of rays is large enough, the realism Space applications require specialized software. For is ensured at subpixel level. Providing the models are instance the PANGU [4, 5] utility is commonly used in accurate enough, SurRender images are virtually the European space sector. Our team at Airbus has been undistinguishable from actual photographs. developing since 2011 the image rendering software In its standard implementation SurRender produces SurRender, which addresses the specific challenges of visible (scattered light) images (although it is potentially physical image simulation (raytracing) with high level of able to make thermal infrared images, see Sec. 6.1). In representativeness for space scenes. Even if it is based on addition to images, useful output is generated such as classical 3D rendering techniques, it adapts those to depth maps (range imaging) and normal maps (slope specific space properties: high variability of objects mapping, hazard detection and avoidance). Active optical sizes, very high range between them, specific optical sensors such as LiDARs can also be modelled with properties. The software recently went through a formal SurRender (Sec. 2.4). qualification process, it has been used in many R&D and technological development projects for ESA (JUICE, GENEVIS), CNES, for the European Community Sun (NEOShield-2) and internal projects (e.g. SpaceTug). Section 2 of this paper presents the methods that Light compose the SurRender software, from first principles, to Object 1 computational implementation. In section 3, we source sampling introduce the user interfaces, the embedded modelling Image Optics language SuMoL and sensor models. Section 4 presents a plane variety of tests that have been made for its formal (sensor) validation. In Section 5 we demonstrate the performances Object 2 of SurRender by focusing on the example of a Moon Light bounces landing application. In section 6 we list additional examples of applications. Finally in Section 7 we discuss Fig. 1: Principle of backward
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